The need for performant tactile sensors in robotics has grown in the past ten years with the emergence of very sophisticated robots and prehension devices. Experience has shown that tactile sensors sensitive to forces exerted thereupon such as pressure forces, torque or shear forces have many pitfalls and do not perform satisfactorily in most applications.
Position sensors used as pointing devices for pointing objects on a screen of a computer are also increasingly found in the computer industry in particular in connection with personal computers (desktop or laptop computers). The pointing devices heretofore known come in numerous forms and perform satisfactorily in most cases. Yet, they also have undesirable features inherent to the physical principles underlying their functioning.
Magnetic tactile sensors have been known to the robotics industry and are based on sensing variations of a magnetic field generated by the sensor or the object to be sensed. In particular, a shear-sensitive magnetoresistive tactile sensor has recently been disclosed in the article published in "IEEE TRANSACTIONS ON MAGNETIC", vol. Mag-22, No. 5, September 1986, entitled "Shear-sensitive Magnetoresistive Robotic Tactile Sensor", by T. J. Nelson et al.
As set forth in the above-referenced article, there is disclosed a tactile sensor consisting of a small magnetized rod, constrained to pivot about its attachment to a thin elastic sheet. Magnetoresistive detectors that produce differential outputs proportional to the x and y displacements of the end of the rod are arrayed underneath the sheet. The rod can pivot about its axis in response to shear forces on the active surface. The difference in resistance of opposite pairs of detectors indicates the angular displacement and thus the shear. This arrangement seems to be performant for determining the shear components of a force but is hardly versatile as an all-function tactile sensor. In particular, the tactile sensor disclosed in the aforementioned article does not appear adequately adapted to sense pressure or torque forces.
More tactile sensors are described in an article by Leon Harmon, published in Recent Advances in Robotics, edited by Beni and Hackwood, 1985 p. 389-424. Because of the unusual characteristics of tactile sensing and the difficult problems encountered in the realization of robust multiple-sensors, the state of the art has developed slowly and seems primitive despite the immense progress in electronics and information processing. The concept of compliance in particular is an important component of tactile sensing. It is also essential that a tactile sensor be proprioceptive, i.e., be able to generate a feedback proportional to the force exerted by the grip of the mechanical arm or wrist.
The instantaneous kinematic and static characteristics of robotic wrist joints are also analyzed in an article entitled "Kinematic and Static Characterization of Wrist Joints and Their Optimal Design", published in IEEE, International Conference on Robotics and Automation, P. 244-250, by H. Asada et al. It is demonstrated in this article that if wrist joints are required to rotate in all directions, the mobility is therefore considered to be optimal in isotropic kinematics.
As to pointing devices, they are now well known in the computer industry and they all serve the same purpose: pointing an object on the CRT terminal of a computer with the maximum accuracy, speed and the least fatigue by the user of the pointing device. A review of the pointing device currently used in the computer industry can be found in an article by Cary Lu, published in "High Technology", January 1984, pages 61 to 65, entitled "Computer Pointing Devices: Living with Mice". Very briefly summarized, the following pointing devices are presently found in the market:
cursor keys: moving the cursor via the keyboard is quite clumsy for text editing, but hopeless for graphics;
joydisks: wherein a disk can be pressed in eight directions each time activating one of a series of peripheral switches;
cursor disks: these devices consist of capacitances sensitive to the pressure of the fingers and the change of capacitance caused thereby;
touch screens: touch-sensitive screens locate the finger of the user and displace the cursor to the position indicated by that finger;
lightpens: the lightpen, which contains a light receptor, is activated by pressing it against the CRT terminal; the receptor detects the scanning beam; a timing circuit compares the beam against the scanning raster and locates the pen's position
touch pads: they consist of two membranes coated with a resistive layer and oriented at right angles. When these membranes touch, the two-axis resistance gives the location of the cursor.
digitizer tablets: tablets are particularly suited for precision drawing and transferring existing designs. They consist of electromagnetic arrays on the surface of which a stylus or cross-hairs are moved.
mice: the two principal types are mechanically driven balls and optically sensed grids. Optical mice operate by sending square-wave trains of information to the CPU dependent on the velocity and the direction of motion. An optical mouse usually comprises a rubber-coated steel ball which contacts two capstans, each connected to an interrupter wheel. Motion along the mouse's X axis rotates one of the wheels and motion along the Y axis rotates the other wheel. Most systems use quadrature coding, a scheme that compares the output of two sensors for each axis of motion. Because of a small offset in the sensor location, the lead or lag on one sensor with respect to the other give the direction.
trackballs: these are essentially upside-down ball-driven mice and are not adequate for precise two-dimensional positioning.
joysticks: most joysticks are mechanically coupled to a pair of potentiometers. They are widely used in computer games but are of clumsy use in business and professional software. They also exhibit variable torque requirements depending upon the direction of tilt.
All the pointing devices hereabove described have specific applications and have undesirable features such as slowness, inaccuracy or unreliability. An egonomic study carried out by Stuart K. Card et al., published in "Ergonomics", 1978, vol. 21, No. 8, pages 601-613 and entitled "Evaluation of Mouse, Rate-Controlled Isomeric Joystick, Step Keys, and Text Keys for Text Selection on a CRT" has shown that of the four devices tested, the mouse is clearly the superior device for text selection on a CRT. The positioning time of the mouse is fast, its error rate significantly lower and its rate of movement nearly maximal with respect to the information processing capabilities of the eye-hand guidance system.
Although mice are presently the most attractive and performant pointing devices, they require a surface that is often unavailable in crowded offices and untidy desks. They also are simply inoperative for laptop computers. Mice are also not performant for small distances, such as moving to one character at a time. Drawing with a mouse is less satisfactory than with a pencil-shaped stylus.
The need has thus developed of a position sensor which measures rotations and would be used mainly in robotic and pointing device applications. Previous attempts to improve the existing devices have proven either ineffective or simply unfeasible. It has therefore drawn the attention of the inventor to design a sensor based on principles not implemented in the existing devices.
The inventor is unaware of applications utilizing the novel arrangements more fully described hereinbelow, nor applications wherein the above-described prior art has been successfully implemented.